Cardiac muscle contraction is regulated by Ca2+ through the troponin complex that consists of three protein subunits: troponin C, troponin I, and troponin T (TnT). This research project investigates the role of a novel posttranslational modification of cardiac TnT (cTnT) in myocardial adaptation to energetic crisis. We recently reported a restricted N-terminal proteolysis of cTnT in myocardial ischemia-reperfusion (Zhang et al., Biochemistry 45:11681-94, 2006) and pressure overload (Feng et al., J. Physiol. 586:3537-50, 2008). This modification selectively removes the N-terminal variable region of cTnT but keeps the conserved middle and C-terminal regions intact. Initial studies in transgenic mouse hearts showed that the N-terminal truncated cTnT (cTnT-ND) remains functional in the myofilaments and alters myocardial contractility. This finding has lead to a hypothesis that the proteolytic removal of the N-terminal variable region of cTnT produces a functional state in cardiac muscle thin filaments as acute adaptation to energetic crisis. To test this hypothesis and understand the physiological and pathophysiological significance of cTnT- ND, three Specific Aims are proposed in this research project: Aim I will characterize the effects of the selective deletion of the N-terminal variable region on the function of cTnT. We will examine the interactions of cTnT-ND with other thin filament regulatory proteins and its functional impacts on the Ca2+ activation of myofibril ATPase and the contractility of cardiac muscle. Aim II will study the role of N-terminal truncated cTnT in compensating cardiac function and providing myocardial protection against ischemia-reperfusion injury. The acute and chronic effects of cTnT-ND on heart function and its role in overcoming energetic crisis will be investigated in ex vivo working hearts and in vivo. Aim III will investigate the role of mechanical stretch of the cardiac muscle in inducing the restricted proteolytic cleavage of cTnT N-terminal segment in order to understand the mechanisms that regulates this posttranslational regulation. Using integrated experimental systems, this study will gain new knowledge for the structure-function relationships of TnT and lay a foundation for future development of new treatment of ischemic heart disease and heart failure. PUBLIC HEALTH RELEVANCE: Myocardial contraction is essential for heart function. The contraction of cardiac muscle is regulated by calcium via the function of troponin, a protein complex in muscle cells. Cardiac troponin T (cTnT) is a subunit of the troponin complex in cardiac muscle. In a recent study, we found a restricted cleavage of cTnT in stress conditions such as acute myocardial ischemia. This modification of cTnT selectively removes the N-terminal variable region and keeps the remaining conserved structure in the myofibrils with functional consequences. Initial studies of transgenic mouse hearts showed that the N-terminal truncated cTnT (cTnT- ND) has a beneficial effect on the pumping efficiency of the heart. This observation lead us to propose a novel hypothesis that the removal of the N-terminal segment of cTnT is a rapid posttranslational mechanism to produce a transient functional state in the cardiac muscle for coping with energetic crisis. In the present study, we will characterize the physiological and pathophysiological functions of cTnT-ND. We shall examine the effects of restricted N-terminal truncation on the biochemical function of cTnT, the calcium activation of myofibril motors and the contractility of cardiac muscle. The potential role of cTnT-ND in reducing myocardial dysfunction and protecting the cardiac muscle during ischemia will be studied in isolated working hearts and in living animals. The mechanisms for mechanical stretch to induce the production of cTnT-ND will also be investigated. These experiments will provide new information for understanding the role of cTnT-ND in myocardial adaptation to energetic crisis. Using integrated physiological systems, this experimental research will also contribute key information for the structure-function relationship of TnT and lay a foundation for future development of new treatment of ischemic heart diseases and heart failure.